The phrase "Sometimes God sends an ex back into your life" implies that there may be a divine or predetermined aspect to these reunions, as if a higher power is orchestrating the circumstances.
The humor in the statement lies in the suggestion that the return of an ex-partner is a test of one's wisdom and growth. It humorously insinuates that repeating the same mistakes with an ex might be an indication of not having learned from past experiences.
In essence, this quote playfully comments on the complexity of relationships and the potential for personal growth and change. It serves as a reminder that revisiting past relationships can provide an opportunity for self-reflection and growth, rather than repeating past errors.